Field of the Invention
The invention relates to a wireless receiver that receives a radio wave from, for example, a wireless microphone and demodulates an audio signal, and in particular relates to a wireless receiver capable of obtaining stable reception characteristics in multi-wave operation in which a plurality of wireless microphones is simultaneously used.
Description of the Related Art
A wireless microphone and a wireless receiver are normally used in one-to-one relationship.
That is, in multi-wave operation in which a plurality of wireless microphones is simultaneously used on a stage or the like, the same number of wireless receivers tuned to transmission frequencies of the respective wireless microphones are used.
In this case, according to Japanese Radio Law, for example, as a frequency band that can be used for a type B wireless microphone, 30 channels are assigned for a specified low power radio device, in which a range of 806.125 to 809.750 MHz is equally divided in units of 0.125 MHz.
Therefore, in a case where the plurality of wireless microphones is simultaneously used as described above, it is necessary to set a transmission frequency of each of the wireless microphones so that the wireless receiver is not influenced by co-channel interference, adjacent-channel interference, or intermodulation interference, and also operation is necessary to match a reception frequency of the wireless receiver to be paired with the transmission frequency of each of the wireless microphones.
However, even when the above-described countermeasures are taken, for example, in a case where the wireless microphones on the stage are close to each other, or in a case where the plurality of wireless microphones simultaneously approaches the antenna of the wireless receiver (in a case of a strong electric field), the wireless receiver is easily influenced by the above-described interference.
As a result, the wireless receiver may be subjected to interference in which a radio wave of a desired reception frequency is suppressed, and interference also occurs in which a noise level of a demodulated signal is increased due to influence of an interference wave.
Accordingly, a conventional wireless receiver includes a muting circuit (also referred to as a squelch circuit) in order to remove audibly unpleasant noise that is generated when being subjected to the above-described interference. As for the muting circuit, a noise muting circuit that opens and closes a signal line according to a noise level in a modulated audio signal, and an RF muting circuit that opens and closes the signal line according to a carrier strength level have been proposed, and are disclosed in patent documents, for example, JP 11-205169 A, or JP 07-202733 A.